I asked about repotting because the pot it's currently in looks rather large for the size of plant. If it had white powder over the leaves a while back, that does sound like powdery mildew, but that can be caused by dryness at the root.
It's hard to tell what's going on, between the white deposits and general sickly appearance, but inspect the stems thoroughly - I think I can see a little black bump on one of them, and it may be there is a scale infestation - on the leaves, these may form white deposits. Check the back of the leaves too, with a magnifying glass if necessary.
If you find scale, spray the whole plant thoroughly with neem oil spray, including under the leaves. Information here on scale and other pest problems with Ivy
The other thing is to get the watering right - if the rootball of the plant is much smaller than the soil surrounding it, it's quite difficult to keep the roots moist. Water when the surface of the soil feels just about dry to the touch, water thoroughly and allow excess water to drain away freely from the base of the pot. Empty the outer tray of the pot after 30 minutes, and again 30 minutes later if more water has collected, so the plant is not left sitting in water.
You might also consider cutting back the topgrowth by a third, to just above a leaf on each stem. This should trigger new growth from the roots over time, and will encourage the plant to bush out more.